Ah but we all have to go sometime, no matter how well we've performed in the past...I'd be interested to know the actual figures in relation to expenditure for modernising, presuming their condemned boiler was very old.
My point being, sometimes the outlay takes decades to recoup in fuel savings. Double glazing isn't always the investment people think and loft insulation can take twenty years if you have to pay for it.
My system is open, modern pressurised systems would most likely require the replacement of the majority if not all of my system. Yes it would be cheaper to run, but at what cost to replace and maintain?! Modern boilers really can't last as long as much older versions and are often much less reliable.
It really depends on the condition of your rads. If they're still ok but just need a flush then you might get away with you or the engineer/plumber removing them and flushing them outside. Converting to a sealed system is a straightforward re-piping job and removal of the header tank(s), we had this done at our last house.